The present invention has particular application in the heat treating of drill blanks, wherein the shank portion of the drill blank must be marked with identifying indicia, such as the size thereof and the name of the manufacturer. Prior to the instant invention, special processes were utilized in the heat treatment of drill blanks in order to maintain the shank of the blank in soft enough conditon not only to imprint the identifying indicia or lettering thereon, but also to enable a drill chuck to effect a suitable gripping action thereon in use of the drill. The convenitonal technique in heat treating drill blanks was in the use of a furnace that employed a salt bath for hardening the fluted portions of the blanks, the shanks of the drills usually being held in insulated grip tongs while the flutes were lowered into the hot salt. After the heat treatment, the flutes were quenched in air or in a lower temperature salt or in oil. After the quenching operation a cleaning procedure such as sand blasting was necessary for the cleaning before tempering and marking of the drill shanks.
The salt bath process has been found objectionable because of the high costs of power involved in maintaining the salt in proper condition for heat treating, and usually the salt bath must be maintained in continuous operation even when the heat treating operation is not being carried out, since it is not economically feasible to cut off the power to the salt bath unit. This technique has also been found to violate certain government regulations now in practice.
Other techniques for heat treating drill blanks have also been used that included a salt bath that heat treated the blanks throughout the length thereof. After the heat treatment operation and the cooling of the blanks, they were subject to a second treatment for softening the shanks. This treatment usually consisted of fixing the blanks in tongs and then lowering the shanks thereof into hot lead or salt. Thereafter a cleaning procedure was also necessary before further processing.
Some prior processes have avoided the use of the salt bath and have employed an atmosphere controlled furnace, the drill blanks being loaded in trays or baskets and then placed in the furnace for the heat treatment thereof. After the heat treatment operation and the passage of the work load into a cooling chamber, the drill blanks were file-hard all over. Although the drill blanks were clean after this heat treatment procedure, they still have to be processed further to soften the shanks which caused discoloration of the shanks and also necessitated a separate cleaning step.
Another method and apparatus for heat treating drill blanks whereby the shanks of the drill blanks were protected during the heat treating operation is disclosed in U.S. Pat. No. 3,704,871; and, although the process as disclosed in this patent protected the shanks by means of a ceramic fiber work holder and achieved the desired result without the use of the salt bath or the other time consuming techniques set forth hereinabove, the process was carried out on a batch basis and was not automatic.
Although all of the prior known procedures accomplish the purpose of enabling the drill shank to be marked with appropriate identifying indicia and to be sufficiently soft for gripping by a chuck, the additional steps involved and the fact that none of the prior procedures was performed on an automatic basis, necessarily increased the cost of manufacture of the drill blanks and further caused considerable delays in obtaining the finished product.